In the printing recording field. technological progress has recently been made in all phases from the impact recording system which uses conventional pressure-sensitive ribbons to the nonimpact recording system, and in the background of such progress there have been accomplishments of the following requirements:
1. printer's cost reduction, PA0 2. removal of printer noise, and PA0 3. alleviation of the maintenance.
Among those nonimpact recording methods, the heat-sensitive transfer recording method attracts attention particularly in respect of being excellent not only in the above-mentioned requirements but also in the resulting image stability. reliability, and the like. However, it is also true that the existing heat-sensitive transfer recording methods are still not adequate.
The remaining most important question is the reduction of the running cost. Upon this, a demand has been made for the development of a heat-sensitive transfer recording medium which is repeatedly usable unlike conventional ribbons for only one time use.
Several techniques for this purpose have until now been disclosed which include those as described in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. publication) Nos. 68253/1979, 105579/1980, 16091/1982, 185192/1982, and the like. Those disclosed in these publications relate to techniques which enable the repetitive printing by how little thermofusible ink can be transferred.
That is, Japanese Patent O.P.I. Publication No. 68253/1979 discloses a method in which a volatile solvent is used to form a resin-component fine porous layer and from the resin component as a solid mother material an ink is thermally fused to be exuded. Japanese Patent O.P.I. Publication No. 105579/1980 discloses a method for the incorporation of a thermofusible ink into a heat-resistant ink layer having a fine porous reticular structure likewise, in which, for example, a polymer resistant against a temperature of more than 120.degree. C. serves as a solid mother material. Japanese Patent O.P.I. Publication No. 160691/1982 describes a method in which an inorganic or organic fine powder (such as carbon black) is used as a solid mother material and from the material a solvent dye as a colored ink is exuded. And Japanese Patent O.P.I. Publication No. 18192/1982 discloses a method in which an ink-impregnated porous paper is used as a solid mother material and from the material the ink is thermally diffused to be exuded.
These techniques each is of an idea that a thermally exudable ink is made present in a porous solid mother material, whereby the ink is little by little exuded to be transferred therefrom. Such the idea is of the utilization of those pressure-sensitive carbon paper techniques as disclosed in, e.g., Japanese Patent Examined Publication No. 13426/1960, for heat-transferable carbon paper.
In the case of a heat-sensitive transfer, however, the presence of the solid mother material used for the purpose of little by little transferring an ink may become a different disadvantage at the same time; that is, the mother material is not substantially fused at the time of the application of energy, or is not mixed with a thermofusible low-melting material, and as a result, the solid mother material itself is substantially nontransferable, and the incorporation of such the nontransferable material in an ink layer causes the whole ink layer to be thick, thus leading to the deterioration of the sensitivity, deterioration of the transfer efficiency, lowering of the transferred image density, overload of the thermal head, or deterioration of the printing quality such as resolution.
It is therefore necessary to develop a repeatedly usable medium which uses no solid mother material and which is free from the above-mentioned disadvantages.
Firstly, the fundamental requirement for making the medium repeatedly usable is that both ink layer and support thereof must be so adequately adhesive that the whole ink of the ink layer can not be transferred at least at once.
Regarding this, formerly, in the pressure-sensitive sheet, and in recent years, also in the field of the heat-sensitive transfer process, those techniques for interposing an adhesion layer as disclosed in Japanese Patent O.P.I. Publication Nos. 68253/1979, 105579/1980, 36698/1982, 96992/1984, etc., have been used. These techniques can be technology for satisfying the fundamental requirement for providing an adequate adhesion between the ink layer and the support.
As aforementioned, however, unless a solid mother material is used to control the transferring amount of the ink, the interposition of an adhesion layer alone is not enough to control the transferring amount of the ink.
That is, only the incorporation of those fundamental materials as generally used in conventional one-time ribbons, including various coloring materials such as various waxes, carbon black. pigments, dyes, etc., low-softening resins, softening agents (oily or semi-solid materials such as castor oil, mineral oil, hydrogenated vegetable oil, lanolin, white vaseline, hydrophilic vaseline, etc.) and the like, in combination into the ink layer is not enough to properly control the transferring amount of the ink.
It is particularly difficult to make the medium repeatedly usable always at a high density according to the change in the surface smoothness of printing paper. For example, even if printing were made a large number of times on a highly smooth paper, in the case of a low-smooth-surface paper, it possibly occurs that the paper requires a large transferring amount of the ink in the first printing so that the printing density in the subsequent printings becomes extremely lowered. Or if an adjustment is made so as to conduct printing properly on a less smooth paper, in the case of a highly smooth paper, there occurs a problem that the transferring amount of the ink in the first printing is too small to obtain an adequate density. Further, there also occurs a problem that the density is low regardless of the smoothness, so that the printing quality is deteriorated.